Impulse drying occurs when a wet paper web passes through the press nip of a pair of rolls in which one of the rolls is heated to a high temperature. A steam layer adjacent to the heated surface grows and displaces water from the sheet in a more efficient manner than conventional evaporative drying. It is projected that wide commercialization of impulse drying would result in very large industry wide energy savings.
In addition to the impact on energy consumption, impulse drying also has an effect on paper sheet structure and properties. Surface fiber conformability and interfiber bonding are enhanced by transient contact with the hot surface of the roll. As the impulse drying process is usually terminated before the sheet is completely dried, internal flash evaporation results in a distinctive density profile through the sheet that is characterized by dense outer layers and a bulky midlayer. For many paper grades, this translates into improved physical properties. The persistent problem with the use of impulse drying, however, is that flash evaporation can result in delamination of the paper sheet. This is particularly a problem with heavy weight grades of paper and it has not been possible to predict under what conditions delamination will occur. This has been a major constraint as to the commercialization of impulse drying.
It has been reported, Crouse, J. W. et al, "Delamination: A Stumbling Block to Implementation of Impulse Drying Technology for Liner Board", Tappi Engineering Conference, Atlanta, Ga., Sept. 13, 1989, that various degrees of delamination were experienced with liner board dried at press roll surface temperatures above 150.degree. C. (300.degree. F.). When delamination was avoided by operating at the lowest limit, water removal efficiencies were not significantly different than those obtained by conventional drying. It was concluded in this report that to realize the potential of impulse drying it would be necessary to alleviate delamination.
In laboratory scale simulations, Lavery, H. P., "High Intensity Drying Processes-Impulse Drying Report" Three DOE/CE/40738-T3, February 1988, it was found that increased pulp refining encouraged delamination and it was postulated that very thick or highly refined sheets exhibit greater resistance to the flow of vapor than thin or coarse paper webs. Hence, if the flow resistance of the web became so large that high pressure steam could not escape, the sheet may not be strong enough to sustain the pressurized vapor and delamination would occur.
The effect of hot surface materials on delamination has been investigated, Santkuyl, R. J., "The Effect of Hot Surface Material on Delamination in Impulse Drying", Master's Program, Institute of Paper Science and Technology, March 1989. Using an electrohydraulic impulse drying press simulator, carbon steel, aluminum and sintered porous stainless steel platens were tested in terms of their ability to dewater and suppress delamination. A felt back-up pad was used in the simulations. It was observed that a difference in thermal diffusivity between steel (1.1.times.10.sup.-5 m.sup.2 /s) and aluminum (6.8.times.10.sup.-5 m.sup.2 /s) had no affect on dewatering capacity or the propensity for paper sheets to delaminate. Porous stainless steel (thermal diffusivity of 2.times.10.sup.-6 m.sup.2 /s) platens provided completely suppressed delamination, although also providing considerably lower dewatering capacity. For porous materials, such as sintered porous stainless steel, a mass balance on the paper sheet showed that a large fraction of the water was removed as vapor and a much smaller fraction was displaced as liquid water into the backup felt. It was concluded that the porous platens do not operate by an impulse drying mechanism. Instead, steam formation and venting at the hot platen-vapor interface augmented by hot pressing were considered to be responsible for water removal. As a resulting of venting, measured temperatures within the vapor sheets never exceeded 100.degree. C. (212.degree. F.) and flash evaporation could not occur.
Accordingly, it is a principal object of the present invention to provide a roll surface material which is suitable for use in impulse drying over a broad range of temperatures and nip residence times but wherein delamination of the paper web is prevented.
It is another object of the present invention to provide a roll surface material that can be heated for impulse drying and can attain efficiencies comparable to that of solid steel rolls but which do not result in delamination of the paper web under high energy transfer conditions.